MOLECULAR EVOLUTION OF THE GLOBIN GENE-CLUSTER-E IN 2 DISTANTLY RELATED MIDGES, CHIRONOMUS-PALLIDIVITTATUS AND CHIRONOMUS-THUMMI-THUMMI

We have studied the evolutionary dynamics of a cluster of insect globi n genes by comparing the organization and sequence of the gene group i n two distantly related species, Chironomus pallidivittatus and C. t. thummi. Although the general architecture of the globin gene cluster h as been conserved, we have found an additional, previously undescribed gene (named Cpa F) in C. pallidivittatus which shows signs of acceler ated sequence evolution at nonsynonymous codon positions. This new gen e is clearly functional, as demonstrated by Northern analysis. Compari son of paralogous and orthologous genes reveals patterns of intraspeci fic sequence homogenization. The head-to-head-oriented globin 3 and 4 gene pairs in C. t. thummi and the gb 4 gene pair in C. pallidivittatu s have been efficiently homogenized, probably by gene conversion, in t heir promoter and coding regions. Inverted transcriptional orientation seems to favor efficient conversion. The orthologous genes from C. t. thummi and C. pallidivittatus reveal different levels of sequence con servation, ranging from 85.3 to 94.7% amino acid identity. Surprisingl y, globin gene E, for which up to now no corresponding protein has bee n detected in the larval hemolymph of C. t. thummi, shows the highest degree of interspecies sequence conservation. This points to an essent ial, as yet unknown function of this globin. The usefulness of globin gene comparisons for dating speciation events in Chironomus is discuss ed.